Is this with the camera level; i.e. not pointing upwards or downwards?
I'm assuming this is the full-frame; i.e. no cropping?
If the answers are, as I suspect - 'Yes' and 'Yes', I cannot see any other explanation than the fact there's some sort of problem with the lens.
Okay, it is at 17mm, but on a crop camera (50D). The distortion seems much more extreme at the left-hand side - and that shouldn't be the case.
I acknowledge my own limited technical knowledge. But I can't see any other rational explanation.
One of my earlier Tamron lenses had the very rear element come loose and I started getting some results similar (though a bit different as the lens was not a prime but a small telephoto 28-70 I think). Tamron said it was due to operator error and refused to fix it, so I just chucked it in the trash and to date, it will be a cold day in hades before I ever buy another Tamron product.
I have previously mentioned that I had been a bit dissatisfied with the results from my Canon 24-105 L lens and odd perspective angles, similar to this was one problem.
Eventually I sent it to a repairer who striped it down and replaced the helicoid and rear barrel assembly; which is along the lines of what Chris said. As far as I can tell so far, there has been a considerable improvement. It cost me a little over £200.
Has your lens had a hard knock, Steve?
Lenses are made to fall within certain performance brackets
What you have is a 17mm lens that also zooms, keeps its very large aperture throughout that range, weighs very little, is pretty darn sharp and costs a few hundred quid - the flip side of that is distortion....lots of distortion - see here.
As an example of what can be achieved when final cost to the consumer is less important - see here.
Even the 17-40mm f4 is a bit on the bendy side at 17mm and that has a 'L' badge on it and a less demanding max aperture.
Sorry but I don't think your lens is faulty you just have to live within its limitations and get to know the lens correction bit of Photoshop like its your very best friend. To double check take a shot with the camera absolutely level (use the spirit level on a good tripod or a separate one on the hot-shoe of the camera itself) to get it bang on.
Not as far as I know Geoff, but I didn't expect to be having to do so much correction, it is really hard to do and takes time.Has your lens had a hard knock, Steve?
Cheers for that Robin; I bought the lens because at certain apertures it could nearly match L type but at 2250 LW/PH nearly match nyquist.
Nearly seems to be a long way.